Liquid crystal materials are investigated not only for applications in display media such as display panels represented by a TN type and a STN type, using reversibility of molecular motions of liquid crystals, but also for applications in optical anisotropic materials such as a retardation plate, a polarizing plate, a light polarizing prism, various light filters, and the like, by use of properties of liquid crystals such as molecular orientation and anisotropy in physical properties such as refractive index, dielectric constant, magnetic susceptibility, and the like.
The optical anisotropic material is obtained, for example, by a process where a liquid crystal compound with a polymerizable functional group or a composition containing the compound is first uniformly oriented in a liquid crystal state. Thereafter, the compound or composition is photopolymerized, while retaining the liquid crystal state, by irradiating with an energy ray such as ultraviolet light and the like to obtain a polymerized film with the uniform orientation state semi-permanently fixed.
In a composition used for this polymerized film, when the temperature of exhibition of the liquid crystal phase is high, unintended thermal polymerization may be induced in addition to the photopolymerization induced by energy rays. Thus, uniform orientation of the liquid crystal molecules may be lost and it becomes difficult to fix the desired molecular orientation. Accordingly, in order to make control of the temperature easier during curing, a polymerizable composition which shows a liquid crystal phase at around room temperature is required.
Further, the polymerized film is obtained by coating the polymerizable composition on a substrate and polymerizing the composition. However, when a nonpolymerizable compound is contained therein, there arise such drawbacks that strength of the polymerized film obtained is insufficient and that strain remains inside the film. In addition, when the nonpolymerizable compound is removed therefrom by a solvent and the like, a problem arises that uniformity of the film cannot be maintained and surface irregularity is generated. Therefore, in order to obtain a polymerized film of uniform thickness, a process of coating a solution of the polymerizable composition in a solvent is preferably employed. The liquid crystal compound or the composition containing the same is thus required to have good solubility in solvents.
As a composition used for the polymerized film, for example, Patent Document 1 reports an optical anisotropic material using a nematic composition. However, even though the film is uniform immediately after curing, the nematic composition described in Patent Document 1 has problems that it lacks thermal resistance and solvent resistance, and that the film deforms and optical properties deteriorate with passage of time.
In Patent Documents 2 to 5, the present inventors have reported compositions which show excellent performance such as thermal resistance, solvent resistance, solubility in solvents, a high glass transition temperature, an exhibition of a liquid crystal phase at low temperature, and the like.
Generally, when the polymerized film is thick, there are problems that orientation control of the liquid crystal molecules contained in the polymerizable composition is difficult, light transmittance deteriorates, and discoloration appears. On the other hand, when a thin polymerized film is prepared, even though excellent orientation control is obtained all over the film, the film thickness becomes difficult to control resulting in a non-uniform surface and, also, crystallization of the film becomes easy to occur. Thus, heretofore known compositions have been unsatisfactory in terms of physical properties and optical characteristics of the thinned polymerized film.
Patent Document 1: Japanese Patent Laid-Open Publication No. H8-3111
Patent Document 2: Japanese Patent Laid-Open Publication No. H10-87565
Patent Document 3: Japanese Patent Laid-Open Publication No. H11-130729
Patent Document 4: Japanese Patent Laid-Open Publication No. 2005-263789
Patent Document 5: Japanese Patent Laid-Open Publication No. 2005-309255